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4.2.4 Else

The ‘#else’ directive can be added to a conditional to provide alternative text to be used if the condition fails. This is what it looks like:

#if expression
#else /* Not expression */
#endif /* Not expression */

If expression is nonzero, the text-if-true is included and the text-if-false is skipped. If expression is zero, the opposite happens.

You can use ‘#else’ with ‘#ifdef’ and ‘#ifndef’, too.